ALL PARTS OF THE GUITAR ARE EQUALLY IMPORTANT

All parts of the guitar are equally important, but some of the areas of particular interest are (from the bottom up) the bridge, strings, neck, fret, and tuning pegs.
Let's begin with the bridge. This is where your stings will be changed. There are round pegs at the end of every string. These will need to be pulled out and new stings inserted. You will need to learn the string names.

Beginning from the top (biggest) string to the bottom string (smallest): EADGBE. The top and bottom strings are the same note, just different octaves.

On the neck of your guitar you should notice two things. One, there are going to be a series of metal lines that extend from one end of the neck to the other. These are called frets. Two, if you look down the side of the neck of your guitar you will see dots. The dots are located at frets 3, 5, 7, 9, 12, 15, and 17. Fret 12 has a double dot for which I will explain further once we get to that portion of these lessons.
Let's talk for a minute about the tuning pegs at the top of the guitar on the headstock. This is where the stings end. Before you ever change strings, pay particular attention to which peg each string ends up.

Here is a helpful hint for remembering how the strings are placed into the tuning pegs. If you are holding your guitar in posture (which will be discussed a little later) there will be three pegs pointing toward the ceiling (upright) and three pointing toward the ground (downward). If you are looking at the guitar as the diagram indicates, there will be three pegs on the left and three pegs on the right. Let's work with the latter example. Low E (biggest string) goes into bottom left peg, A goes to middle left peg, D goes to top left peg, G does to top right peg, B goes to middle right peg, high E (smallest string) goes to bottom right peg

Here is an important reminder for you once you have reached this point. Be sure that you thread all strings the same way. What I mean by this is be sure that when you turn the peg clockwise, it tightens the string while counter-clockwise loosens the string. One thing you can try, and I suggest, is that you change out one string at a time. Do not just take out all of the strings and go for it. This is where mistakes happen when trying to get your strings in the correct place and through the tuning pegs. Pay attention to what you are doing so that you don't find your guitar in a mess.